If prune juice will help on constipation, how much and how often should the prune be drunk?

Prune juice contains sugars that are not absorbed easily by the gut. They also bind water which helps keep nearby poop moist. The effect is dose related, meaning the more you drink the more likely you will have moist or loose poop. How much to drink is not specific and varies with the concentration of the juice. You can self-test & decide if it helps & how much is needed. You can overdo it.